Water in tight places

A clutch of studies this week on water in confined geometries. Alan Soper and his coworkers at ISIS have used neutron scattering to look at the structure of water within the hydropholic pores of Vycor glass, the walls of which are lined with surface OH groups [J. Phys. Chem. B 111, 5610 (2007)]. For pores 40 angstroms wide – several dozen molecular diameters – they find significant disruption of the bulk liquid structure. The average number of hydrogen bonds per molecule is decreased from about 3.6 to about 2.2, and they see structural changes extending at least two layers into the liquid owing to the orientational effects of hydrogen-bonding to surface OH. It’s perhaps a little surprising that the effect is so big, and certainly raises questions about how bulk-like cell water is (the average distance between macromolecules in the cytoplasm is just 1-2 nm).

Sow-Hsin Chen at MIT and his colleagues have continued to study deeply supercooled water. Confined in mesoporous silica with pore diameters of 15 angstroms, it can be supercooled to at least 160 K. Deeply supercooled water is predicted to have a density minimum around 70 K below the well-known density maximum at 277 K – an echo of the ice-like low-density liquid phase predicted under pressure, and of the low-density amorphous ice that has been well established already. Using SANS, Chen and colleagues now see this density minimum at around 210 K for D2O [PNAS early edition, www.pnas.org/cgi/doi/10.1073/pnas.0701352104].

In a preprint [arxiv:0705.2348], Simone Melchionna use MD simulations to look at water in hydrophobic pores. They find spontaneous cavity formation for pore diameters of around 2 nm, which they relate to previous predictions and reports of density depletion and drying at hydrophobic surfaces, particularly the Lum-Chandler-Weeks model. In narrower pores (around 1.5 nm or so), this cavitation can result in complete (but intermittent) emptying, as has been proposed in protein ion or water channels as a gating mechanism. (In those cases, the precise nature of the residues in the pore neck seems to be rather crucial.)

Finally, something with a real biomolecule in it. Gerhard Hummer and colleagues have conducted MD simulations of water within the nonpolar cavities of tetrabrachion, a protein of the hyperthermophilic archaebacterium Staphylothermus marinus [JACS asap, doi:10.1021/ja070456h]. This contains several large hydrophobic cavities linked by a central channel, and the crystal structure shows that all contain water. The researchers find that the largest cavity contains 7-9 water molecules both at room temperature and at 365 K, the organism’s optimal growth temperature. But it empties at a slightly higher temperature, around 384 K. The second-largest cavity is filled with five waters at room temperature, as the X-ray structure confirms, but this breaks up at 365 K. Thus, both cavities are close to emptying at 365 K, and Hummer et al. suggest that this emptying might create sockets into which the proteases, which bind to it in its functional state, can plug.

More Hofmeister head-scratching

A paper by Dér et al. [J. Phys. Chem. B, doi:10.1021/jp066206p] makes the bold claim of providing a general microscopic interpretation of Hofmeister effects – the ion-specific salting-in or salting-out of proteins. I’m not sure that it succeeds. The basic idea is that the ions induce changes to the protein-water interfacial tension: so-called kosmotropes make the interface more hydrophobic, and chaotropes make it more hydrophilic. I can’t help feeling that the paper is hampered from the outset by an insistence on retaining the chaotrope/kosmotrope terminology, which was coined to suggest that the respective ions ‘break’ or ‘make’ ‘water structure’. There’s no evidence that ions do either, at least in terms of exerting any global influence on the hydrogen-bonded network. Indeed, Dér et al. acknowledge that spectroscopic studies [Omta et al., Science 301, 347; 2003] show no change in hydrogen-bonding on addition of ions beyond their first hydration shell. As far as I can make out, they seem to say that preferential segregation of ions at or away from the interface means that localized effects on H-bonding can be specifically felt there. But it’s not clear to me what they are thinking of in alluding to this surface segregation of ions – there’s no reference, for example, to the studies of that (at the air-water interface) by Jungwirth, Saykally and others. Nor do the authors seem to take into account how this picture applies to hydrophobic surfaces (Bruce Berne has studied this, and found ion-specific segregation). In any event, what results seems rather unsatisfactory, since we are then left with the confusing chao/kosmotrope terminology but a hint that in fact all the action is taking place at the interface (which is probably the case) – and an attempt to explain that action in terms of macroscopic interfacial tensions (which are not known anyway for protein-water interfaces). I can’t help thinking that it remains more useful to think more explicitly about how ions might modify the nature of the microscopic protein-water interface, and how this picture changes when two surfaces come together (so that, say, adsorbed ions are excluded).

Greg Voth’s group has just published a very nice review on proton transport in aqueous systems, including all the work on ‘water wires’ in aquaporin, M2, cytochromes and other proteins [J. Phys. Chem. B 111, 4300-4314; 2007].

Sylvia McLain and colleagues have conducted an extensive neutron-scattering study of the structure of proline solutions [J. Phys. Chem. B 111, 45568-4580; 2007]. Proline acts as an osmolyte or osmoprotectant, apparently protecting proteins against denaturation under water stress. It has been suggested that this is somehow due to proline clustering in solution, but McLain et al. find no strong evidence of that. Indeed, proline seems barely to perturb water’s hydrogen-bonded network at all, while remaining sufficiently hydrated to attain good solubility. They speculate that, despite the only weak tendency of prolines to cluster, they might form a protective sheath around proteins, chaperoning them to prevent denaturation.
Update: McLain and colleagues have just published a comparison of these experimental results with computer simulations (which show reasonable agreement): J. Phys. Chem. B 111, 8210; 2007.